1. Field of the Invention
The present invention relates to digital communication systems and, more particularly, to baseband amplitude limiting in code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) communication systems.
2. Prior Art
Many digital communications systems utilize amplitude and phase modulation, producing complex baseband signals, which are pulse shaped before modulation to the carrier frequency. The complex, baseband signals are typically represented as an in-phase signal (I) and a quadrature signal (Q). For many modulation formats and multiple access schemes, these complex baseband signals have large peak values relative to the average signal value. When the peak-to-average power ratio of the baseband signal is large, the signal requires reduction in the power amplifier to avoid saturation. The peak-to-average power ratio limits the average power that can be transmitted due to the finite maximum output power of the power amplifier in the RF front-end (RFFE). Alternatively, for a fixed average transmit power, the peak to average power ratio will determine the required maximum output power of the RF power amplifier. In both cases, a decreased peak-to-average power ratio is desirable.
To decrease the peak-to-average power ratio of the signal, many digital communications systems employ clipping, also termed amplitude limiting, on the baseband complex signal. Clipping suppresses the signal peaks, reducing the peak-to-average power ratio of the signal with minimal distortion to the signal. The most common and effective form of clipping is circular clipping, which preserves the angle of the complex signal while limiting the maximum magnitude. Circular clipping follows the input-output relation,
      I    out    =      {                                                                      I                in                                                                    :                                                                                                          I                    in                    2                                    +                                      Q                    in                    2                                                  ≤                                  Pp                  out                                                                                                                          I                  in                                ·                                                                            Pp                      out                                        ⁢                                          /                                        ⁢                                          (                                                                        I                          in                          2                                                +                                                  Q                          in                          2                                                                    )                                                                                                                          :                                                                                                          I                    in                    2                                    +                                      Q                    in                    2                                                  >                                  Pp                  out                                                                    ⁢                                  ⁢                  Q          out                    =              {                                                            Q                in                                                                    :                                                                                                          I                    in                    2                                    +                                      Q                    in                    2                                                  ≤                                  Pp                  out                                                                                                                          Q                  in                                ·                                                                            Pp                      out                                        ⁢                                          /                                        ⁢                                          (                                                                        I                          in                          2                                                +                                                  Q                          in                          2                                                                    )                                                                                                                          :                                                                                                          I                    in                    2                                    +                                      Q                    in                    2                                                  >                                  Pp                  out                                                                        
In hardware implementations, circular clipping is often implemented using a look-up table (LUT). A LUT implementation avoids the need to compute the constellation magnitude, implement the square root, and division functions as are done in U.S. Pat. No. 6,266,320 to Hedberg et al.; such functions are complex operations for digital hardware. With a LUT-implementation, the LUT holds the output I and Q (Iout,Qout) values for all possible combinations of input values (Iin, Qin). When the number of possible input I and Q values is small, the LUT approach is attractive. However, as the number of possible I and Q input values increases the size of the memory used to store the LUT must grow proportionately. With 10-bit input I and Q values and 8-bit I and Q output values, the LUT would require a memory of size 2,097,152 bytes. If the I/Q constellation is symmetric, then the table may be reduced by one-forth to 524,288 bytes, but is still quite large. In many applications a LUT-based circular clipper may be impractical when the number of possible input I and Q values is large.
Therefore, it is desirable to provide an efficient method and system to decrease a peak-to-average power ratio of a communications signal before the signal is amplified by a power amplifier.